Lorenz Number for High-Purity Copper

Following a recent report that the Lorenz number, $L=\frac{\lambda }{\sigma T}$, does not approximate to the theoretical value of $2.45\times {10}^{-8\mathrm{}}W$ ohm ${\mathrm{deg}}^{-2\mathrm{}}$ for very pure copper at the lowest temperatures, measurements have been made of thermal and electrical conductivities on copper of similar purity. Between 2 and 4°K, values of $L$ are observed lying in the range ${2.5}_4$ to ${2.4}_0\times {10}^{-8\mathrm{}}W$ ohm ${\mathrm{deg}}^{-2\mathrm{}}$.